sandbox: Unprotect DATA regions in bus tests

On my Ubuntu 18.04.1 machine two driver-model bus tests have started
failing recently. The problem appears to be that the DATA region of the
executable is protected. This does not seem correct, but perhaps there
is a reason.

To work around it, unprotect the regions in these tests before accessing
them.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/os.h

@@ -334,4 +334,16 @@ void os_localtime(struct rtc_time *rt);
  * os_abort() - Raise SIGABRT to exit sandbox (e.g. to debugger)
  */
 void os_abort(void);
+
+/**
+ * os_mprotect_allow() - Remove write-protection on a region of memory
+ *
+ * The start and length will be page-aligned before use.
+ *
+ * @start:	Region start
+ * @len:	Region length in bytes
+ * @return 0 if OK, -1 on error from mprotect()
+ */
+int os_mprotect_allow(void *start, size_t len);
+
 #endif